The present invention relates to a method of depositing an electronically conductive interconnection layer on an electrode of an electrochemical cell.
High temperature electrochemical cells are well known. In these types of cells, typified by fuel cells, a porous support tube of calcia stabilized zirconia, has an air electrode cathode deposited on it. The air electrode may be made of, for example, oxides of the perovskite family, such as doped lanthanum manganite. Surrounding the major portion of the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia. A selected radial segment of the air electrode is covered by an interconnection material. The interconnection material may be made of a doped lanthanum chromite film. The generally used dopant is Mg, although Ca and Sr have also been suggested.
Both the electrolyte and interconnect material are applied on top of the air electrode by a modified electro-chemical vapor deposition process, at temperatures of up to 1450.degree. C., with the suggested use of vaporized halides of zirconium and yttrium for the electrolyte, and vaporized halides of lanthanum, chromium, and magnesium, or calcium or strontium for the interconnection material.
It would be economically desirable to form at least the interconnect material by a simple sintering process which would employ less expensive equipment and use low cost oxides or chemicals to form the desired interconnection.
In U.S. Pat. No. 4,631,238 (Ruka), a Co and/or Mg doped lanthanum chromite interconnection was described. Means of making the interconnection were generally described as including vapor deposition and traditional sintering techniques.
An improved method of bonding fuel cell interconnections was taught in U.S. Pat. No. 4,861,345 (Bowker et al.), where particles of lanthanum chromite, doped with at least one of Sr, Mg, Ca, Ba and Co, and having on each particle surface a coating of CaO+Cr.sub.2 O.sub.3, were placed on an air electrode surface and heated in air without any applied pressure. The Ca and Cr coated on the surfaces of the individual particles were incorporated into the structure of the lanthanum chromite. This system allowed formation of sintered interconnections without cracking the fragile air electrode by pressure techniques. A slurry of the particles in a Ca(NO.sub.3).sub.2 +Cr(NO.sub.3).sub.3 solution was applied to the air electrode by brushing or tape casting. Heating then formed the layer on the particles. Further heating caused the CaO+Cr.sub.2 O.sub.3 to melt and flow into voids between the particles and ultimate reduction of void volume in the interconnection. This invention required particle coating, and resulted in a small, open porosity. Even a small open porosity is troublesome for fuel cell operation and life.
What is needed is a convenient method to make lanthanum chromite interconnections without open porosity on air electrodes. It is one of the objects of the invention to provide such a method and to provide such an interconnection on a fuel cell.